Matrix decoders for quadraphonic sound system

ABSTRACT

In a multi-channel sound system, decoding apparatus for deriving from two composite signals transduced from a two-track record medium a plurality (e.g., four) of separate sound signals contained in the composite signals for presentation on respective loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. Typically, the two composite signals respectively contain signal components intended for presentation on loudspeakers positioned at the left front and right front corners of a listening area, and both composite signals contain signals intended for presentation on loudspeakers positioned at the left back and right back corners of the listening area, with the left back and right back signals in one composite signal in substantially quadrature relationship with corresponding signals in the other. The decoder includes all-pass phase-shifting networks designed to transmit all frequencies in the frequency range of interest which are operative to shift the phase of one of the composite signals relative to the other by an angle to cause corresponding signals in the relatively phase-shifted composite signals to be either in phase or in phase opposition and networks for combining the relatively phase-shifted composite signals to derive the four separate sound signals.

United States Patent 1191 Bauer 111 3,835,255 [451 Sept. 10,1974

[ MATRIX DECODERS FOR QUADRAPHONIC SOUND SYSTEM [75] Inventor: Benjamin B. Bauer, Stamford,

Conn.

22 Filed: Mar. 7, 1973 21 Appl. No.: 338,691

Related US. Application Data [60] Continuation-in-part of Ser. No. 185,050, Sept. 30, 1971, Pat. No. 3,813,494, which is a division of Ser. Nos. 44,224, June 8, 1970, abandoned, and Ser. No. 118,271, Feb. 24, 1971, Pat. No. 3,784,744, and Ser. No. 124,135, March 15, 1971.

[52] US. Cl 179/1 GQ, 179/l00.4 ST,

151] Int. Cl. .LIT -110411'5/00 58 Field ofSearch.. 179/1 6, 1 GP, 1 G0, 15 BT, 179/100.4 ST, 100.1 TD

[56] References Cited UNITED STATES PATENTS 3,564,162 2/1971 Bauer 179/1 G 3,632,886 l/l972 Scheiber 179/1 GQ 3,646,574 2/1972 l-lolzer..... 179/1 G 3,746,792 7/1973 Scheiber 179/1 00 OTHER PUBLlCATlONS rived from Three Microphones by Klipsch IRE Transactions on Audio, Man-April, 1959. Four Channels and Compatibility by Scheibe, AES Preprint Oct. 15, 1970.

Primary Examiner-Kathleen H. Claffy Assistant Examiner-Thomas DAmico Attorney, Agent, or FirmSpencer E. Olson [5 7] ABSTRACT In a multi-channel sound system, decoding apparatus for deriving from two composite signals transduced from a two-track recordmedium a plurality (e.g., four) of separate sound signals contained in the composite signals for presentation on respective loudspeakers to give the listener the illusion of sound coming from a corresponding number of separate sources. Typically, the two composite signals respectively con tain signal components intended for presentation on loudspeakers positioned at the left front and right front comers of a listening area, and both composite signals contain signals intended for presentation on loudspeakers positioned at the left back and right back comers of the listening area, with the left back and right back signals in one composite signal in substantially quadrature relationship with corresponding signals in the other. The decoder includes all-pass phase-shifting networks designed to transmit all frequencies in the frequency range of interest which are operative to shift the phase of one of the composite signals relative to the other by an angle to cause corresponding signals in the relatively phase-shifted composite signals to be either in phase or in phase opposition and networks for combining the relatively phaseshifted composite signals to derive the four separate sound signals.

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MATRIX DECODERS FOR QUADRAPHONIC SOUND SYSTEM This is a continuation-in-part of application Ser. No. 185,050 filed on Sept. 30, 1971, now U.S. Pat. No. 3,813,494, which in turn is a division of now abandoned application Ser. No. 44,224 filed June 8, 1970, and of application Ser. No. 118,271 filed on Feb. 24, 1971, now US. Pat. No. 3,784,744 and of application Ser. No. 124,135 filed Mar. 15, 1971, all in the name of Benjamin B. Bauer.

CROSS-REFERENCE TO OTHER APPLICATIONS This invention is related to the subject matter of the following other co-pending applications, all of which are assigned to the assignee of the present invention and application: Ser. No. 164,675 filed July 21, 1971 as a continuation-in-part of now abandoned application Ser. No. 40,510 filed May 26, 1970; Ser. No. 44,196 filed June 8, 1970, now US. Pat. No. 3,708,631; Ser. No. 155,976 filed June 23, l97lnow U.S. Pat. No. 3,798,373; and Ser. No. 328,814 filed FEEB. 10, 1973 now abandoned.

BACKGROUND OF THE INVENTION There is an increasing interest in multiple-channel recording and reproduction because of the variety of sounds and music fonns that can be achieved thereby. The modern stereophonic phonograph is capable of recording, or encoding, modulation along two separate channels, and it is usual practice to include a third, or center, channel by matrixing or combining it as an in-phase phantom channel with the other two, which causes it to be recorded as lateral modulation parallel to the record surface. Upon reproduction, the third (or center) channel appears on the two loudspeakers of the stereophonic phonograph, with equal loudness and inphase relationship, and an observer placed centrally between the loudspeakers perceives the illusion of the third channel being located between the other two. The fourth, or vertical, channel when reproduced on a conventional two-loudspeaker stereophonic phonograph gives the illusion of unlocalized sound. Although there have been attempts to reproduce the third or center channel on a separate loudspeaker, the results have not been entirely satisfactory, and most stereophonic systems, even though many stereo records carry a center" channel, employ only two loudspeakers.

In the aforementioned co-pending application Ser. No. 164,675 of William S. Bachman, there is described a system for providing third and fourth playback channels to otherwise two-channel systems by feeding third and fourth loudspeakers with signals respectively representing the sum and difference between the left and right channel signals. The left and right loudspeakers may be located, for example, on opposite sides of a listening area, with the loudspeakers for the two virtual channels positioned at opposite ends of the listening area. Each loudspeaker displays the particular infonnation fed to its channel accompanied by half-power signals from its adjacent channels. This system provides a pseudo-four-channel effect, but does not give a complete illusion of each channel appearing independently on its corresponding loudspeaker.

A better illusion of each channel appearing independently on its corresponding loudspeaker is provided by the system described in the above-mentioned US. Pat.

No. 3,708,631 which includes four gain control amplifiers through which the four separate channels of information are respectively applied to corresponding loudspeakers, and a logic control circuit which derives its signals from the left and right output terminals of the transducer for automatically controlling the gain control amplifiers to enhance the realism of four separate channels of information. While this system has some drawbacks, it provides a significant improvement in the art of reproduction of multi-channel sound.

In stereophonic practice the two loudspeakers are normally placed in two adjacent corners of the listening area, and it is conventional in the reproduction of fourchannel recordings to have the four sources originate from the four corners of the listening area. However, the systems described in the aforementioned copending application Ser. No. 164,675 and US. Pat. No. 3,708,631 are designed to preserve symmetry with loudspeakers placed centrally of the four walls of the listening room. If the loudspeakers were placed in the corners, the aspect of the originally recorded sound would be shifted by 45, causing an inconsistency confusing to the listener. Also, since there are practical difficulties in finding suitable locations for loudspeakers centrally of the walls in most homes, it is preferable that the reproducing system permit the placement of the loudspeakers at the corners of the listening room.

SUMMARY OF THE INVENTION A general object of the present invention is to provide methods and apparatus for combining four channels of program information, edited for presentation on four loudspeakers placed at the comers of a listening area, into two composite signals suitable for presentation over the two loudspeakers of stereophonic playback apparatus which at the same time will admit of decoding into four separate signals corresponding to the four signals originally encoded, for presentation on corresponding sound-reproducing devices. The four channels of information for convenience identified as L; for left front, R for right front, L,, for left back and R, for right back, are combined to form two composite signals L (left total) and R (right total) having the following characteristics:

1. The L; signal appears only in the L composite signal and the R, signal appears only in the R composite signal; thus the front signals are completely isolated from each other. When L and R are applied to a stereophonic cutter, L; produces a 45 modulation, while R, produces a 45 modulation, precisely as with conventional stereo.

2. The left back (L signal appears in both the L and R composite signals at reduced amplitude, 0.707 in the preferred embodiment, and in quadrature with each other, with the L, component in the L composite signal preferably leading the corresponding signal in the R signal. This causes a stereophonic cutter stylus to described a circular motion in the clockwise direction, which when combined with the lengthwise motion of the groove becomes a clockwise helix.

3. The right back (R signal also appears in both the L and R composite signals at reduced amplitude, 0.707 R in the preferred embodiment, and in quadrature with each other. The R component in the R signal leads the corresponding component in the L signal which causes the cutter stylus to describe a circular motion in the counterclockwise direction.

A more specific object of the present invention is to provide decoding apparatus for deriving the four original signalsfrom the two composite signals. The decoder includes all-pass phase-shifting networks designed to transmit all frequencies in the audio frequency range of interest which are operative to shift the phase of one of the composite signals relative to the other by an angle of substantially 90 to cause corresponding signals in the two composite signals to be either in phase or in phase opposition. The relatively phase-shifted composite signals are selectively combined to derive a pair of output signals respectively containing the left back and right back signals as predominant components, and the composite signals, in which the left front and right front signals are respectively predominant, constitute the other two out put signals.

BRIEF DESCRIPTION OF THE DRAWING An understanding of the foregoing and additional aspects of this invention may be gained from consideration of the following detailed description, taken in conjunction with the accompanying drawing, in which:

FIG. 1 is a schematic diagram of an encoder for combining a plurality of independent signals intended for ultimate display on four separate loudspeakers;

FIG. 2 is a diagram useful in explaining the operation of the circuit of FIG. 1;

FIG. 3 is a schematic diagram of one form of matrix decoder embodying the invention;

FIG. 4 is a plan view of a listening area illustrating the location of four loudspeakers therein and the phasor diagrams of the signals appearing thereat;

FIG. 5 is a schematic diagram of another form of matrix decoder according to the invention;

FIG. 6 is a schematic diagram of still another form of matrix decoder according to the invention;

FIG. 7 is a diagram useful in explaining the operation of the decoder of FIG. 6;

FIG. 8 is a schematic diagram of another embodiment of matrix encoder embodying the invention; and

FIG. 9 is a diagram useful in explaining the operation of the decoder of FIG. 8.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, there is schematically illustrated one form of encoding apparatus for combining four independent signals, intended for ultimate display on four separate loudspeakers, into two composite signals for recording or transmission on a two-track medium, such as a stereophonic disc record or a two-track tape. The encoder has four input terminals 40, 42, 44 and 46 for respectively receiving four separate signals which, for convenience, are designated left front (L left back (L,,),'right back (R and right front (R the locations in a listening area of the four loudspeakers on which the signals are intended for ultimate presentation. These signals are represented by vertical arrows of equal length which signify, for purposes of the analysis to follow, that the input signals are assumed to be of equal magnitude and referred to the same phase reference. The encoder includes six all-pass phase-shift networks 48, 50,52, 54,56 and 58 designed, to introduce a substantially constant phase shift to the applied signal over the frequency range of interest without altering their magnitudes. Each of the networks has a reference phase shift (1!, which is a function of frequency, the two phase shifters 50 and 56 introducing only the reference phase shift. Phase shifters 48 and 58 provide a phase shift equal to Ill 45, and networks 52 and 54 provide a phase shift equal to 41 It is to be noted that according to the herein used convention the phase-shift angles produced by the phase-shifters 48-58 are lagging angles; that is, a network with a phase-shift of (1!; 90) produces an output lagging 90 behind that produced by a network with a phase-shift of (i1: 0).

The signals L; and R,, respectively identified with the left front and right front" loudspeakers, are applied via their respective terminals 40 and 46 through their associated all-pass networks 48 and 58 to respective summing circuits 60 and 62. The left back signal L is applied to both of phase-shift networks 50 and 52, the output signal from the former being applied to summing circuit 60 with attenuation corresponding to the multiplicand 0.707, and the output signal from network 52 is applied to summing circuit 62 with the same attenuation. The right back signal R is applied to both of phase-shift networks 54 and 56, the output signals from which are respectively applied with 0.707 attenuation to summing circuits 60 and 62. The summing circuits 60 and 62, which are of conventional design and well known to ones skilled in the art, are operative to produce respective composite signals L and R at their corresponding output terminals 64 and 66. These signals may be applied to the left and right terminals of a stereophonic disc record cutter, for example, or to the two recording heads of a two track tape recording apparatus, or to any other known two-track medium, in a manner which will be apparent to ones skilled in the art.

Although the matrixing apparatus has been thus far described in terms of four input signals, if it is desired to have a signal appear centrally in the reproducing system, a center signal, designated by the arrow labeled C, may be applied equally and in phase to terminals 40 and 46, or to the terminals 42 and 44, or to all four terminals simultaneously, as indicated by the curved arrows. It will be evident that the C signal will be subjected to the phase shift of those of networks to which it is applied, in the example of FIG. 7 to networks 48-58 and will become part of the composite signals L and R The nature of the composite signals appearing at terminals 64 and 66 will be seen from the phasor diagrams adjacent the terminals. It is seen that each of these signals contains a predominant frontloudspeaker signal, L, and R,', respectively, both of which are shifted in phase relative to input signals L; and R; by (tl: 45). The L signal further includes signals L and R, at 90 to each other, with the L, signal leading, and in a 45 relationship with L}. The C signal appears as C in both composite signals in the same relative phase position as the signals L, and R The R signal contains the signal R;', and also the two signals L and R, at 90 to each other. It is important to note, however, that L and R, are reversed in phase relative to their positions in the L signal, with R leading and signal L lagging relative to the corresponding signals on terminal 64. As noted earlier, however, the

signal C is again in the same relative position with respect to the corresponding signal on terminal 64.

Another significant feature of the composite signals L and R is that the L,, signal in one of the composite signals is in quadrature with the L signal in the other, and that the R signals in the two composite signals are also in quadrature.

Since the signals L, and R usually will be incoherent signals, if recorded on a stereophonic disc record they will appear independently as separate modulations of the left and right channels. The signals C being in phase at both of terminals 64 and 66 will cause lateral modulation of the disc record. The fact that signal L,, at terminal 64 leads the L, at terminal 66 by 9 will cause the stylus of a stereophoniccutter to describe a circular motion in a clockwise direction. Similarly, because signal R,,' at terminal 64 lags behind signal R at terminal 66 by 90 will cause circular motion of the stylus in a counter-clockwise direction. Thus, it is seen that the five signals applied to the matrix system of FIG. 1 may be applied to a stereophonic disc record as five distinct types of modulations, namely, modulation of the left and right walls of the groove, lateral modulation, and clockwise and counter-clockwise helical modulation.

The form of modulation on the disc record, as viewed from the point of view of the cutter tip, looking in the direction of motion of the groove, is illustrated in FIG. 2. The L, signal causes motion at 45 to the horizontal, the R, signal causes motion at 45 to the horizontal, and the C signal causes lateral or horizontal modula tion. These three modulations, it will be recognized, are identical with those which obtain in the cutting of a conventional stereophonic record. As a significant departure from conventional practice, there is, additionally, clockwise circular modulation L corresponding to the left back loudspeaker signal, and the counterclockwise circular modulation R corresponding to the right back loudspeaker signal. Since the L,, and R modulations have a significant horizontal component (as projected on the line C) it is evident that they will produce equivalent signal components in the horizontal mode, therefore assuring full compatability with a monophonic phonograph player. An important advantage of the just-described method of combining the input signals to form the two composite signals is that the stereophonic record or tape recorded in this manner can be replayed over any stereophonic or monophonic player with full and complete reproduction of all of the sounds recorded on the record.

Although one form of encoding apparatus has been described to illustrate how the input signals are combined to form the composite signals L and R as background for the description to follow of apparatus for decoding the composite signals, the encoder may take a variety of forms and still produce two composite sig nals possessing the essential features of both containing both the L and R signals, with the L and R signals in one in quadrature relationship with the corresponding signals in the other, and with the L,, signal leading the R signal in one and lagging the R signal in the other. The encoder of FIG. 1 and several alternative embodiments are disclosed and claimed in the aforementioned application Ser. No. 328,814.

Referring now to FIG. 3, there is illustrated one form of apparatus for deriving from the two composite signals described above four separate signals for presentation on corresponding loudspeakers. The signals L and R are derived from a stereophonic record, for example, by a suitable transducer (e.g., a conventional stereophonic pickup) and are applied to input terminals 70 and 72, respectively, of the decoder apparatus. These signals are first passed through respective allpass phase-shift networks 74 and 76, the latter providing a reference phase shift of til and network 74 providing a phase shift of 011 90). The value of lll may be the same as the reference phase shift ill used in the encoding apparatus of FIG. 1, or it may be different, the principal requirement being that the reference phase angle #1 be the same in both networks 74 and 76. The signals L and R appearing at the output terminals of the phase-shift networks differ from the signals L and R only in that they are displaced by 90 with respect to each other. This is illustrated by the phasor diagrams in FIG. 3 in which the phasors of the R signal are in the same relative position as in the R signal,

but the vectors of the L signal are all rotated 90 clockwise. Since the reference phase shifts ill, are the same in both networks, their relative effects have been disregarded in the phasor diagrams. An important consequence of the 90 relative phase-shift of the two composite signals is that the L components in the L and R signals are now in phase with each other, and the R components are in phase opposition.

The composite signal L which contains L, as its predominant component, is applied through an additional all-phase phase-shift network 78, which provides a reference phase shift 41 without changing the ampli tude of the signal, and thence through a gain control amplifier 80 (whose gain may be controlled by applying a control signal to the amplifier) and a suitable power amplifier to a loudspeaker 82. Similarly, the composite signal R in which the signal R," is predominant, is passed through an all-pass phase-shifter 84, designed to provide a phase shift of (\I1 90), and then through gain control amplifier 86 and a power amplifier to loud speaker 88. A signal containing'the left back signal L as its predominant component is derived by summing the L and R signals, each after multiplication by the factor 0.707, in a summing circuit 90. Being in phase opposition, the R signals are canceled, with the result that the L signal is accompanied by L, and

R each reduced in amplitude by the factor 0.707.

The output signal from summing circuit is applied to an all-pass phase-shifter 92' which introduces a relative phase shift of (111 90), and thence through gain control amplifier 94, and a suitable power amplifier, to loudspeaker 96. To obtain the fourth output signal, containing the right back signal, R,,", as its predominant compartment the L and R signals are summed in a similar summing circuit 98 after multiplicantion by the factors 0.707 and 0.707, respectively, whereby the L signals are canceled and the R signal is accompanied by reduced amplitude signals R," and L,". This sum (really difference) signal is passed through phase-shift network 100, which provides a reference phase shift and is then applied to gain control amplifier 102 and thence through a power amplifier to loudspeaker 104.

The gains of amplifiers 80, 94, 102 and 86 may be controlled by a control and switching logic 106 in response to signals derived from the output terminals of phase-shifters 74 and 76 in the manner described in the aforementioned US. Pat. No. 3,708,631 or in US. Pat.

No. 3,7 84,744. Briefiy, the control and switching logic 106 is operative to recognize the channel or channels having the dominant signal among L,", R,", L," and R and applies control signals to the gain control amplifiers which increase-the gain of the channel or channels containing the instantaneously dominant signal or signals and to reduce the gain of the other channels to give a substantially perfect illusion of four separate independent sources of sound. As the sound diminishes in the channel first identified and another sound appears on a different channel, the logic circuit functions to rapidly attenuate the gain in the first channel and to increase the gain in a different channel.

It should be mentioned that the all-pass phaseshifters employed in the circuits of FIGS. 1 and 3 may be of any design that will provide the relatively constant angular differences specified in the drawing. Usually, the function I!) should be as small as possible, consistent with the provision of the necessary differential phase shift. The phase-shift function should be smooth, without any rapid changes of phase angles as a function of frequency, as such rapid changes might cause changes in the timbre of the sound being recorded and- /or reproduced. It should also be noted that the sense of rotation of the respective helical modulations can be reversed in the recording process by inverting the relative phase shifts of the L, and R signals in the encoder of FIG. 1. This would result in the phasor R in the left channel leading the phasor R in the right channel, and would cause the phasor L in the right channel to lead the phasor L, in the left channel. It will be recognized that if this were done, it would be necessary to interchange the all-pass networks 74 and 76 in order to obtain correct decoding of the four individual signals.

FIG. 4 illustrates the location in a listening room or area of the loudspeakers 82, 96, 104 and 88 for optimum display of the signals decoded by the system of FIG. 9. Specifically, loudspeakers 82, 88, 96 and 104 are placed in positions corresponding to the left front, right front, left back and right back, respectively. Phasor diagrams of the signals appearing on each of these loudspeakers are presented adjacent their respective loudspeaker. It will be observed that the signals L R,", L," and R are predominant at loudspeakers 82, 88, 96 and 104, respectively; the signals from two other channels appearing in each of the main channels are about three db lower in level than the principal signals and, accordingly, tend not to be prominent in the mind of the listener. Rather, he will hear primarily the four independent channels being presented on the four loudspeakers.

It has been observed that phase shifting networks 78, 92, 100 and 84, while contributing to the audible channel separationof the four-loudspeaker system, can be dispensed with and still obtain acceptable channel separation.

Referring now to FIG. 5, there is shown in schematic form an alternative decoder in which a simplification of the circuitry is achieved along with an improvement in performance. It will be remembered that in order for the circuit of FIG. 3 to perform the decoding function it is necessary to first introduce a relative phase shift of 90 with phase-shift networks 74 and 76, which incidentally places the signalsL, and R, in quadrature. This relationship is not undesirable, except when these two phasors contain a common center signal. To form an ideal virtual image of thiscenter signal between the two front loudspeakers, the two front signals should remain in phase. This desirable relationship may be obtained by various stratagems, the one used in the system of FIG. 3 being the incorporation of the four additional ill-networks 78, 84, 92 and 100. In a decoder not intended for audition of the highest quality it is permissible to dispense with these \b-networks, instead connecting the two front loudspeakers (i.e., 82 and 88) to terminals and 71 instead of to the output terminals of phase-shift networks 74 and 76, so that the two front loudspeakers receive the full, unmodified signals L and R respectively, in which the signals L; and R; pre dominate and are in proper phase relationship. However, even if this is done, there is a Ill-function phase angle between the two front loudspeakers and the two rear loudspeakers, resulting in a certain degree of side and back image blurring and dissymetry. This latter effect is not sufficient to appreciably diminish the quality of the quadruphonic image and is the type of compromise which may be accepted in lower cost equipment. In the best professional equipment, however, it is important that the phasor groups be presented in a most favorable phase relationship.

This goal is accomplished in the decoder shown in FIG. 5. This decoder achieves the same relative phase relationship of the phasors applied to the loudspeakers obtained in the embodiment of FIG. 3, but with a saving of two ill-networks; that is, the system of FIG. 5 requires only four, instead of six, ill-networks.

In this decoder, the two composite signals L and R recovered from a two-channel medium, and respectively portrayed by phasor groups 10 and 12, are applied to the input terminals 150 and 152. These signals are in all respects identical to the corresponding signals applied to the input terminals of the decoder of FIG. 3. Unlike the system of FIG. 3, however, in whic two ill-networks are used to properly position the compo nents of the two signals for subsequent combination, the decoder of FIG. 5 utilizes four such networks, 154, 156, 158 and 160, two of which provide a phase shift of (II! 0), and the other two introducing a phase shift of (1!: The L signal is applied to both of networks 154 and 156, and the R signal is applied to both of networks 158 and 160. By reason of the relative 90 phase shift, the phasor groups 162 and 164 appearing at the outputs of networks 154 and 156, corresponding to the L signal, are in quadrature relationship, and similarly, the phasor groups 166 and 168 appearing at the outputs of networks 158 and 160, respectively, corresponding to the R signal, are also in phase quadrature. Because the phase angle \11 generally varies with frequency, the phasor groups 162, 164, 166 and 168 do notbear a fixed angular relationship with respect to phasorgroups 10 and 12, but inasmuch as the reference angle 111 is the same for all of the networks, it is permissible totreat them as bearing a fixed relationship with respect to each other. It will be remembered that the signals L L,,', R, and R, are usually complex program signals and, therefore, the phasor relationships within each phasor group represents the relationships of the same frequency components of the signals.

As in the circuit of FIG. 3, the object of the decoder is to derive from the incoming signals L and R four separate signals predominantly containing the signals 1 L,,', R, and R,', respectively, and to reproduce them over respective loudspeakers 170, 172, 174 and 176. To this end, the signals represented by phasor out change to loudspeakers 170 and 176, respectively,

after amplification by respective gain control amplifiers 178 and 180 in the event a logic circuit is used for enhancement of channel separation. A signal predomi-v nantly containing L is derived by adding 0.707 of the output of ill-network 156 and 0.707 of the output of ill-network 160 in a summing circuit 182, the output signal from which may be represented by the phasor group 184, in which the signal L predominates, with the signals L and R, also being present but at a 3 dB lower level. This signal is amplified by gain control amplifier 186 (when logic is used) and applied to loudspeaker 172. A signal predominantly containing the R signal is obtained by adding in summing circuit 188 0.707 of each of the output signals from til-networks 154 and 158, the resultant signal, represented by phasor group 190, being applied to a respective control amplifier 192 for application to loudspeaker 174. Thus, the loudspeakers 170, 172, 174 and 176 carry signals which have predominant information from the leftfront, left-back, right-back and right-front channels, respectively. As with the decoder of FIG. 3, each of these signals is contaminated with information from two other signals, but the contaminating signals being part of the same original program, their contribution is not unpleasant and, indeed, often provides an improvement in ambience or spaciousness of the musical selection.

It will be noted that the phasor groups 162, 184, 190 and 168 exhibit relative phase-shift relationships identical to the corresponding phasors of FIG. 4 produced by the decoder of FIG. 3, this desired relationship having been obtained with only four ill-networks, whereas the system of FIGT3 requires six ill-networks to achieve the same result. Thus, the decoder of FIG. 5 offers the advantage of greater simplicity, with accompanying reduced cost. The arrangement comprising the four ill-networks and two summing circuits consitutes a satisfactory decoder for connection through suitable amplitiers and loudspeakers to produce a highly realistic and satisfactory rendition of the original quadruphonic program.

As with the decoder of FIG. 3, in the interest of achieving the illusion of greater independence or purity of the decoded signals, it may be desirable to provide a logic and control circuit 196 to provide enhancement of the individual predominant signals. The logic and control circuit disclosed and claimed in U.S. Pat. No. 3,784,744 may be used for this purpose.

Referring now to FIG. 6, there is shown another decoder having the same general form as the system of FIG. 5 but differing in certain respects to facilitate decoding of composite signals having a different form. In this case, the composite signal L applied to input terminal 200 contains a predominant component L a subdominant component 0.707R in phase with L;, and a component 0.707L in quadrature with L;, and the R signal applied to input terminal 202 contains a dominant component R a sub-dominant component 0.707L in phase opposition with R,, and a subdominant component 0.707R,, in quadrature with R Composite signals of this form may be produced by encoders of the kind disclosed in co-pending application Ser. No. 124,135 and claimed in co-pending application Ser. No. 328,814. It will be seen that these composite signals have in common with the composite input signals shown in FIG. 5 the features that the dominant L; and R; signal components are in phase with each other, that the L, component in one composite signal is in quadrature with the L component in the other, and the R component in one composite signal is in quadrature with the R component in the other.

The decoder of FIG. 6 also utilizes four Ill-networks 204, 206, 208 and 210, two of which provide a phase shift of (III 0) and the other two providing a phase shift of (1!! The L signal is applied to both of networks 204 and 206 and the R signal is applied to both of networks 208 and 210. By reason of the relative 90 phase shift, the phasor groups 212 and 214 appearing at the output terminals of networks 204 and 206, corresponding to the 1. signal, are in quadrature relationship, and similarly the phasor groups 216 and 218 appearing at the output terminals of networks 208 and 210, respectively, corresponding to the R signal, ar also in phase quadrature.

The signals represented by phasor groups 212 and 218, which respectively contain L, and R, as their predominant components, are applied without change to output terminals 220 and 222, respectively, and constitute two of the output signals from the decoder. To derive an output signal in which L is the predominant component, signals from the phase-shifting networks 206 and 210 are each multiplied by 0.707, inverted in sign, and summed in a summing junction 224 to produce at output terminal 226 a composite signal represented by the phasor diagram 228. The fourth output signal, in which the R component is predominant, is obtained by multiplying each of the signals from phaseshift networks 204 and 208 by 0.707 and summing them, without inversion, in a summing junction 230 to produce at the output terminal 232 the composite signal represented by phasor diagram 234.

. While the operation of the decoder has thus far been described for corner signals, its action on encoded composite signals resulting from application to the encoder of signals from other angles will be seen from the diagram of FIG. 7 in which the phasor groups 212, 218, 228 and 234 have been expanded. It is seen, for example, that when an L; signal is applied to the encoder (corresponding to an azimuth angle of 315), the signals appearing at the diagonally opposite L, and R terminals, represented by the arrows 240 and 242, respectively, are in phase or additive. The same in-phase relationship of diagonally opposite signals is exhibited when a right back signal (corresponding to an azimuth angle of is applied, as indicated by the arrows 244 and 246. However, when an R; or an L, signal is applied to the encoder, the diagonally opposite signals are in phase opposition, or subtractive, as shown by the pairs of phasor arrows 248 and 250, and 252 and 254, respectively. Thus, application of signals L, and R to the encoder results in stronger decoded signals, especially at bass frequencies, than do R; and L signals. While this effect may occur when the matrix decoder is used by itself, the unwanted or transferred subdominant signals can be significantly diminished by using the blending technique disclosed in U.S. Pat. No. 3,798,373 and/or logic and control circuitry of the kind described in U.S. Pat. No. 3,784,744.

It will also be noted from FIG. 7 that when a center front (C signal is applied to the encoder, resulting in the application of equal 0.707C, signals to the front terminals of the encoder, which appear in phase in the composite signals L and R two in-phase signals depicted by phasors 256 and 258, appear at the front output terminals of the decoder, and two signals in phase opposition, represented by phasors 260 and 262, appear at the back output terminals. While the antiphase signals may be diminished or canceled by the previously mentioned blending technique, or by the action of the front-back logic disclosed in application Ser. No. 155,976 which functions to attenuate the signals represented by phasors 260 and 262 while enhancing the signals represented by phasors 256 and 258, despite such action, or, if, for example, sufficiently strong signals are applied as will cause the transmission through the matrix to be altered by the action of the logic circuit, signals represented by phasors 260 and 262 may be emphasized while those represented by phasors 256 and 258 may be attenuated.

The aforementioned potential anomolies of the decoder of FIG. 6 for certain signals into the encoder are essentially eliminated by the matrix decoder shown in FIG. 8 to the input terminals 200 and 202 of which composite signals L and R of the same form as shown in ljIQ. 6 are respectively applied. As in the decoder of FIG. 6, the L signal is applied to both of a pair of ill-networks 204 and 206 and the R signal is applied to both of a pair of ill-networks 208 and 210, which are operative to produce the relatively phase-shifted output signals depicted by phasor groups 212, 214, 216 and 218, which correspond to the similarly numbered phasor groups in FIG. 6. However, the manner in which these relatively phase shifted signals are combined to derive the four output signals is different. Output signals containing L, and R, as predominant components are taken from the output terminals of phase-shift networks of 206 and 208, respectively, and respectively correspond to the L and R composite signals each shifted in phase by 90; it will be observed, however, that there is no relative phase shiftbetween these at the output terminals. An output signal having L as its predominant component is obtained by multiplying each of the output signals from phase shift networks 206 and 210 by 0.707 and summing them in a junction 270, and an output signal containing R as its predominant component is obtained by multiplying each of the output signals from nil-networks 204 and 208 by 0.707 and adding them in junction 272. The output signals I R;', L, and R, are portrayed by phasor groups 214, 216, 274 and 276, respectively.

The action of the decoder of FIG. 8 for various types of signals applied to the encoder will be seen from the diagram of FIG. 9 in which the phasor groups 214, 216, 274 and 276 are expanded. It is seen that when either of the corner signals, that is, L R,, L, or R,,, (which occur at 315, 45, 225 and 135 azimuth, respectively) is applied to the encoder, there is in each case an in-phase or additive relationship between the main signal and its adjacent transferred signal, while the signal diagonally opposite from the main signal is in quadrature therewith. By virtue of this positioning of the signals, the subtraction effect inherent in the decoder of FIG. 6 is essentially eliminated. Also, the transferred signals corresponding to the main comer signal are of equal amplitude and are in quadrature with respect to each other, and consequentlydo not interfere with the operation of the wave-matching logic described in US. Pat. No. 3,784,744.

Considering now the action of the decoder when a center front (C;) signal is applied to the encoder, resulting in the application of equal in-phase 0.707C; signals to the front terminals of an encoder (0 azimuth) it is seen that the two front signals, as well as both back signals, have the same magnitudes, and the signals in each of the pairs are in phase. In contrast, if a center back (C signal is applied to the back terminals of the encoder, meaning application of two equal, in-phase 0.707C; signals (225 and 135 azimuth, respectively), the signals in the resulting pair of front signals, as well as the signals in the resulting pair of back signals are of equal magnitude but are in phase opposition. This property can be utilized in frontback logic control circuitry of the type disclosed in application Ser. No. 124,135 designed to be operative in response to either or both of the signals of each of the front or back pairs being in-phase to cause attenuation of back signals and enhancement of front signals, and operative in response to either or both of the signals of each pair being in phase opposition to cause attenuation of the front signals and enhancement of back signals.

As has been noted, when a center back (C signal is applied to the encoder, the decoder of FIG. 8 produces center back signals in phase opposition. While this might appear to be an undesirable condition, it is relatively innocuous because in the practice of this stereoquadraphonic system producers are discouraged against applying solo and other important signals to the center back of the quadraphonic array for the reason that such signals, being recorded out-of-phase on a disc record, could not be reproduced monophonically. Any center back signals such as reverberation, instantaneously panning, etc. are likely to be relatively unimportant with the consequence that their appearance in phase opposition is usually unobjectionable. Any weakness in this respect is overshadowed by the fact that application of a strong bass signal to the center front of the encoder will not be diminished in intensity by the decoder when it is accompanied by, say, a side-back signal, because any transfer of signals from front-toback is accompanied by an increase in amplitude of the same signal in the back channel, which are in phase, and consequently will not cause the signal in question to be diminished.

The matrix decoder of FIG. 8 may be used alone, or with logic and control circuitry of the types alluded to above, or it may be used in combination with the blending" feature disclosed in US. Pat. No. 3,798,373. When suitable blend coefficients are used, center front signals in the front channels are emphasized and thissignal is attenuated in the back channels, and at thesame time center back signals in the back channels are emphasized and this signal is attenuated in the front channels;

It will be evident from the'foregoing description that the decoders of FIGS. 3, 5, 6 and 8 each include input circuits including all-pass phase-shifting networks for shifting the phase of one of the composite signals relative to the others by so as to cause corresponding signals in the relatively phase-shifted composite signals to be either in :phase or in phase opposition to enable derivation of output signals containing L and R, as predominant components by simple linear combination of selected relatively phase-shifted composite signals,

and networks for combining the relatively phaseshifted composite signals. The other two output signals,

respectively containing L, and R; as their predominant components, are, in essence, the input composite signals.

1 claim:

1. Apparatus for decoding four individual audio information signals to the extent they are contained in first and second encoded composite signals, each of which contains up to three of said audio information signals, two of which are common in said two composite signals and corresponding ones of said common signals being at a predetermined phase angle relative to each other, said apparatus comprising:

first and second input terminals to which said first and second composite signals are respectively applied,

first, second, third and fourth output channels,

means for coupling said first and second composite signals to said first and second output channels, respectively, with the same relative phase as they exhibit at said input terminals,

all pass phase-shifting means connected to said input terminals for shifting the phase of one of said composite signals relative to the other by said predetermined phase angle over the frequency range of interest for positioning said common signals in one of said relatively phase-shifted composite signals either substantially in phase coincidence or substantially in phase opposition with corresponding ones of said common signals in the other of said relatively phase-shifted composite signals,

means for selectively combining predetermined proportions of said relatively phase-shifted first and second composite signals and operative to produce third and fourth composite signals each containing a different one of said common signals as its predominant component, and

means for coupling said third and fourth composite signals to said third and fourth output channels, respectively.

2.Apparatus in accordance with claim 1, wherein said phase-shifting means comprises first and second all-pass phase-shifting networks operative to shift the phase of said first composite signal relative to said second composite signal by said predetermined phase ang wherein said means for coupling said first and second composite signals to said first and second output channels comprises said first and second phaseshifting networks and third and fourth all-pass phase-shifting networks respectively connected between said first phase-shifting network and said first output channel and between said second phase-shifting network and said second output channel, said third and fourth phase-shifting networks being operative to shift the phase of the phase-shifted first composite signal from said first phase-shifting network relative to the phase-shifted second composite from said second phase-shifting network signal by said predetermined phase angle, and

wherein said means for coupling said third and fourth composite signals to their respective output channels comprises fifth and sixth all-pass phase-shifting networks operative to shift the phase of said third composite signal relative to said fourth composite signal by said predetermined phase angle.

3. Apparatus in accordance with claim 1 wherein said predetermined phase angle is substantially and said predetermined proportion is substantially the decimal fraction 0.707.

4. Apparatus in accordance with claim 2, wherein said predetermined phase angle is substantially 90 and said predetermined proportion is substantially the decimal fraction 0.707.

5. Apparatus in accordance with claim 1, wherein said phase-shifting means comprises first and second pairs of all-pass phase-shifting networks and wherein said first and second composite signals are applied to both phase-shifting networks of its respective pair, the

first network of each pair being operative to shift the phase of signals applied thereto relative to the signals applied to the second network of the pair by said predetermined phase angle,

wherein said means for coupling said first and second composite signals to their respective output channels comprises connections between the first network of the first pair and said first output channel and between the first network of the second pair and said second output channel, and

wherein said combining means comprises a first network for combining said predetermined proportion of the output signal from the second network of the first pair with said predetermined proportion of the output signalfrom the first network of the second pair for producing said third composite signal, and a second network for combining said predetermined proportion of the output signal from the first network of the first pair with said predetermined proportion of the output signal from the second network of the second pair for producing said fourth composite signal.

6. Apparatus in accordance with claim 5 wherein said predetermined phase angle is substantially 90 and wherein said predetermined proportion is substantially the decimal fraction 0.707.

7. Apparatus in accordance with claim 1, wherein said phase-shifting means comprises first and second pairs of all-pass phase-shifting networks each having input and output terminals, the first network of each pair being operative to shift the phase of signals applied thereto by a predetermined reference angle and the second network of each pair being operative to shift the phase of signals applied thereto by an angle which differs from said predetermined angle by 90, means for coupling said first composite signal to the input terminals of both phase-shifting networks of said first pair, and means for coupling said second composite signal to the input terminals of both phaseshifting networks of said second pair, and wherein said combining means comprises first summing means connected to the output terminal of the second phase-shifting network of said first pair and to the output terminal of the first phaseshifting network of said second pair operative to add equal proportions of the signals appearing thereat to produce said third composite signal, and second summing means connected to the output terminal of the first phase-shifting network of said first pair and to the output terminal of the second phase-shifting network of said second pair operative to add equal proportions of the signals appearing thereat to produce said fourth composite signal.

8. Apparatus in accordance with claim 7 wherein said proportion is substantially the decimal fraction 0.707.

9. Apparatus for decoding first and second composite input signals respectively containing dominant signals a and b and each including a sub-dominant signal component c of substantially equal magnitude and in substantially quadrature phase relationship in said first and second composite signals and a sub-dominant signal component d of substantially equal magnitude and in substantially quadrature phase relationship in said first and second composite signals, the signal components and d in one of the composite signals being in leading and lagging relationship, respectively, with the signal components 0 and d in the other of the composite signals, said apparatus comprising:

first and second input circuits having respective first and second input terminals and each including allpass phase-shifting means for deriving from the first and second composite signals first and second composite output signals which are in phase quadrature relationship with'one another,

first, second, third and fourth output channels,

means for coupling the first and second composite signals from the first and second input terminals without relative phase-shift to said first and second ouptut channels, respectively, first summing means for combining equal proportions of the first composite output signal from the first input circuit and of the second composite output signal from the second input circuit to produce a third composite signal predominantly containing the signal component 0,

second summing means for combining equal proportions of the second composite signal from the first input circuit and of the first composite output signal from the second input circuit to produce a fourth composite signal predominantly containing the signal component d, and

means for coupling said third and fourth composite signals to said third and fourth output channels, respectively.

10. Apparatus in accordance with claim 9, wherein said input circuits each include a pair of all-pass phaseshifting networks to both of which the corresponding composite input signal is applied, one of the phaseshifting networks of the pair being operative to shift the phase of the applied signal by a predetermined reference angle and the other phase-shifting network of the pair being operative to shift the phase of signals applied thereto by an angle which differs by 90 from the said reference angle.

11. Apparatus in accordance with claim 10, wherein for each of the composite output signals, the propor-v tion of the signal applied to the first and second summing means is 0.707.

12. Apparatus for decoding first and second input composite signals respectively designated L and R and each including to the extent they are present subdominant left back (L and right back (R component signals with the same predetermined phase-shift angle between said L component signals and between said R component signals and with the L component signal in one of said composite signals leading the L, component signal in the other and with the R component signal in said one composite signal lagging the R component signal in the other, and respectively including to the extent they are present dominant left front (L and right front (Rf) component signals, into four composite output signals respectively containing said Ly, R;, L and R component signals as its predominant signal, said apparatus comprising:

first and second input terminals to which said L and R signals are respectively applied,

first, second, third and fourth output channels respectively identified as L R;, L and R output channels,

means connected between said first and second input terminals and said first and second output channels for coupling said first and second input composite signals respectively containing said L, and R, component signals as predominant signals to said first and second output channels,

means connected to said first and second input terminals including all-pass phase-shifting networks for shifting the phase of one of said input composite signals relative to the other by said predetermined phase-shift angle for positioning the L and R components in one of said relatively phase-shifted composite signals either in phase coincidence or in phase opposition with the L and R components, respectively, in the other relatively phase-shifted composite signal, and

means connected in circuit between said phaseshifting means and said third and fourth output channels for selectively combining predetermined proportions of said relatively phase-shifted first and second composite signals and operative to couple to said third and fourth output channels composite signals which respectively contain said L and R component signals as predominant components.

13. Apparatus in accordance with claim 12, wherein said predetermined phase-shift angle is substantially and wherein said predetermined proportion is substantially the decimal fraction 0.707.

14. Apparatus in accordance with claim 12, wherein said input circuits each include a pair of all-pass phaseshifting networks to both of which the corresponding composite input signal is applied, one of the phaseshifting networks of the pair being operative to shift the phase of the applied signal by a predetermined reference angle and the other phase-shifting network of the pair being operative to shift the phase of signals applied thereto by an angle differing from said reference angle by said predetermined phase-shift angle, and wherein said signal-combining means comprises first summing means for combining equal proportions of the composite signal from the first network of the first pair and the composite signal from the second network of the second pair and coupling to said L output channel a composite signal which contains said L component signal as its predominant component, and

second summing means for combining equal proportions of the composite signal from the second network of the first pair and the composite signal from the first network of the second pair and coupling to said R output channel a composite signal which contains said R component signal as its predominant component.

15. Apparatus in accordance with claim 14, wherein said predetermined phase-shift angle is substantially 90, and

wherein for each of the composite signals from said phase-shifting networks, the proportion of the signal applied to the first and second summing means is 0.707.

16. Apparatus in accordance with claim 14, wherein said predetermined phase-shift angle is substantially 90,

wherein for each of the composite signals from the first network of the first pair and from the second network of the second pair, the proportion of the signal applied to the first summing means is 0.707, and

wherein for each of the composite signals from the second network of the first pair and from the first network of the second pair, the proportion of the signal applied to the second summing means is 0.707.

17. Apparatus in accordance with claim 12, wherein said input circuits each include first and second all-pass phase-shifting networks to both of which the corresponding composite input signal is applied, said first phase-shifting network being operative to shift the phase of the applied signal by a predetermined reference angle and said second phase-shifting network being operative to shift the phase of the applied signal by an angle differing from said reference angle by substantially 90, v

wherein the composite signals from the second network of said first and second input circuits, respectively containing said L, and R, component signals as their predominant component, are respectively coupled to said first and second output channels, and wherein said signal-combining means comprises first summing means for combining substantially 0.707 of the composite signal from said second network of said first input circuit and substantially 0.707 of the composite signal from said first network of said second input circuit and coupling to said third output channel a composite signal which contains said L component signal as its predominant component, and

second summing means for combining substantially 0.707 of the composite signal from said first network of said first input circuit and substantially 0.707 of the composite signal from said second network of said second input circuit and coupling to said fourth output channel a composite signal which contains said R component signal as its predominant component.

18. A method of substantially reproducing at least four individual audio information signals respectively designated left front (Ly), right front (Ry), left back (L and right back (R contained in two information channels, wherein the signals in said channels respectively include one of said L; and R, signals together with predetermined proportions of both said L and R signals with the same predetermined substantially constant phase-shift angle between said L signals and between said R signals such that the L signal in one of said channels leads the L signal in the other. and the R signal in said one channel lags the R signal in the other, comprising the steps of:

coupling the signals in said channels to respective output channels without relative phase shift therebetween to provide two composite output signals respectively containing said L, and R, signals as predominant components,

shifting the phase of the signal in one of said channels relative to the signal in the other channel by said predetermined phase-shift angle for positioning the L and R components in one channel either in phase coincidence or inphase opposition with corresponding components in the other channel, and

selectively combining predetermined equal proportions of said relatively phase-shifted signals to produce third and fourth composite output signals respectively containing said L and R signals as predominant components. I

19. The method according to claim 18, wherein said predetermined phase-shift angle is substantially wherein the signals in the two channels are relatively shifted in phase by substantially 909, and

wherein said predetermined proportion is substantially the decimal fraction 0.707.

20. Apparatus for reproducing at least four individual audio information signals to the extent they are contained in two information channels, wherein the signal in each of said channels comprises a combination of at least three of said audio information signals with preselected phase and amplitude relationships, said apparatus comprising first and second input circuits to which the signals in said two information channels are respectively applied, said input circuits including all-pass phaseshifting means operative to shift the phase of one of said channel signals relative to the other channel signal by a preselected angle which remains substantially constant over the frequency range of said audio information signals thereby to produce relatively phase-shifted first and second channel sig' nals,

electrical circuit means for combining said relatively phase-shifted first and second channel signals with preselected amplitude and phase relationships for deriving a first pair of output signals in each of which a different desired one of two of said audio information signals is predominant, and

circuit means for deriving from said first and second channel signals a second pair of output signals in each of which a different desired one of the other two of said audio information signals is predominant,

whereby a different desired one of said audio information signals is predominant in each of said four output signals.

21. Apparatus in accordance with claim 20 wherein said phase-shifting means is operative to shift the phase of one of said channel signals relative to the other by substantially 90.

22. A decoder for reproducing at least first, second and third of first, second, third and fourth audio information signals to the extent said four audio information signals are contained in first and second composite input signals respectively containing said first and second audio information signals in equal predominant proportions and both containing said third and fourth audio information signals in equal sub-dominant proportions with said third and fourth audio information signals in one of said composite input signals in substantially quadrature relationship with the third and fourth audio information, signals, respectively, in the other composite input signals, said decoder comprising:

means for coupling said first and second composite signals without relative phase shift between them to first and second output terminals, respectively, as first and second output signals in which said first and second audio information signals are respectively predominant,

means for shifting the phase of one of said composite input signals relative to the other by about 90 for positioning the third audio information signal in the phase-shifted one input signal in phase with the third audio information signal in the unshifted other input signal and for positioning the fourth audio information signal in the phase-shifted one input signal in phase opposition with the fourth audio information signal in the unshifted other input signal, and

means for combining said phase-shifted one input signal and said unshifted other input signal to produce at a third output terminal a third output signal in which said third audio information signal is predominant.

23. Apparatus in accordance with claim 22 wherein said predominant and sub-dominant proportions are in the ratio of about 1:.707,

wherein said phase-shifting means comprises first and second all-pass phase-shifting networks to which said first and second composite input signals are respectively applied for shifting the phase of one of said composite input signals relative to the other by about 90, and wherein said combining means comprises first means for adding substantially 0.707 of the output signal from one of said first and second phaseshifting networks to substantially 0.707 of the output signal from the other for producing said third output signal, and

second means for subtracting substantially 0.707 of the output signal from one of said first and second phase-shifting networks from substantially 0.707 of the output signal from the other for producing at a fourth output terminal a fourth output signal in which said fourth audio information signal is predominant.

24. Apparatus in accordance with claim 22, wherein said predominant and sub-dominant proportions are in the ratio of about 1:.707,

wherein said phase-shifting means comprises first and second pairs of all-pass phase-shifting networks, the first network of each pair being operative to shift the phase of signals applied thereto relative to the signals applied to the second network of the pair by about and wherein said first and second composite input signals are applied to both phaseshifting networks of a respective pair, and wherein said combining means comprises first network for combining substantially 0.707 of the output signal from the second phase-shifting network of the first pair with substantially 0.707 of the output signal from the first phase-shifting network of the second pair for producing said third output signal in which said third audio information signal is predominant, and

a second network for combining substantially 0.707 of the output signal from the first phase-shifting network of the first pair with substantially 0.707 of the output signal from the second phase-shifting network of the second pair for producing a fourth output signal at a fourth output terminal in which said fourth audio information signal is predominant.

25. Apparatus in accordance with claim 24, wherein the first network of each of said pairs is operative to shift the phase of signals applied thereto by a predetermined reference angle and the second network of each of said pairs is operative to shift the phase of signals applied thereto by an angle differing from said reference angle by about 90. 

1. Apparatus for decoding four individual audio information signals to the extent they are contained in first and second encoded composite signals, each of which contains up to three of said audio information signals, two of which are common in said two composite signals and corresponding ones of said common signals being at a predetermined phase angle relative to each other, said apparatus comprising: first and second input terminals to which said first and second composite signals are respectively applied, first, second, third and fourth output channels, means for coupling said first and second composite signals to said first and second output channels, respectively, with the same relative phase as they exhibit at said input terminals, all-pass phase-shifting means connected to said input terminals for shifting the phase of one of said composite signals relative to the other by said predetermined phase angle over the frequency range of interest for positioning said common signals in one of said relatively phase-shifted composite signals either substantially in phase coincidence or substantially in phase opposition with corresponding ones of said common signals in the other of said relatively phaseshifted composite signals, means for selectively combining predetermined proportions of said relatively phase-shifted first and second composite signals and operative to produce third and fourth composite signals each containing a different one of said common signals as its predominant component, and means for coupling said third and fourth composite signals to said third and fourth output channels, respectively.
 2. Apparatus in accordance with claim 1, wherein said phase-shifting means comprises first and second all-pass phase-shifting networks operative to shift the phase of said first composite signal relative to said second composite signal by said predetermined phase angle, wherein said means for coupling said first and second composite signals to said first and second output channels comprises said first and second phase-shifting networks and third and fourth all-pass phase-shifting networks respectively connected between said first phase-shifting network and said first output channel and between said second phase-shifting network and said second output channel, said third and fourth phase-shifting networks being operative to shift the phase of the phase-shifted first composite signal from said first phase-shifting network relative to the phase-shifted second composite from said second phase-shifting network signal by said predetermined phase angle, and wherein said means for coupling said third and fourth composite signals to their respective output channels comprises fifth and sixth all-pass phase-shifting networks operative to shift the phase of said third composite signal relative to said fourth composite signal by said predetermined phase angle.
 3. Apparatus in accordance with claim 1 wherein said predetermined phase angle is substantially 90* and said predetermined proportion is substantially the decimal fraction 0.707.
 4. Apparatus in accordance with claim 2, wherein said predetermined phase angle is substantially 90* and said predetermined proportion is substantially the decimal fraction 0.707.
 5. Apparatus in accordance with claim 1, wherein said phase-shifting means comprises first and second pairs of all-pass phase-shifting networks and wherein said first and second composite signals are applied to both phase-shifting networks of its respectIve pair, the first network of each pair being operative to shift the phase of signals applied thereto relative to the signals applied to the second network of the pair by said predetermined phase angle, wherein said means for coupling said first and second composite signals to their respective output channels comprises connections between the first network of the first pair and said first output channel and between the first network of the second pair and said second output channel, and wherein said combining means comprises a first network for combining said predetermined proportion of the output signal from the second network of the first pair with said predetermined proportion of the output signal from the first network of the second pair for producing said third composite signal, and a second network for combining said predetermined proportion of the output signal from the first network of the first pair with said predetermined proportion of the output signal from the second network of the second pair for producing said fourth composite signal.
 6. Apparatus in accordance with claim 5 wherein said predetermined phase angle is substantially 90* and wherein said predetermined proportion is substantially the decimal fraction 0.707.
 7. Apparatus in accordance with claim 1, wherein said phase-shifting means comprises first and second pairs of all-pass phase-shifting networks each having input and output terminals, the first network of each pair being operative to shift the phase of signals applied thereto by a predetermined reference angle and the second network of each pair being operative to shift the phase of signals applied thereto by an angle which differs from said predetermined angle by 90*, means for coupling said first composite signal to the input terminals of both phase-shifting networks of said first pair, and means for coupling said second composite signal to the input terminals of both phase-shifting networks of said second pair, and wherein said combining means comprises first summing means connected to the output terminal of the second phase-shifting network of said first pair and to the output terminal of the first phase-shifting network of said second pair operative to add equal proportions of the signals appearing thereat to produce said third composite signal, and second summing means connected to the output terminal of the first phase-shifting network of said first pair and to the output terminal of the second phase-shifting network of said second pair operative to add equal proportions of the signals appearing thereat to produce said fourth composite signal.
 8. Apparatus in accordance with claim 7 wherein said proportion is substantially the decimal fraction 0.707.
 9. Apparatus for decoding first and second composite input signals respectively containing dominant signals a and b and each including a sub-dominant signal component c of substantially equal magnitude and in substantially quadrature phase relationship in said first and second composite signals and a sub-dominant signal component d of substantially equal magnitude and in substantially quadrature phase relationship in said first and second composite signals, the signal components c and d in one of the composite signals being in leading and lagging relationship, respectively, with the signal components c and d in the other of the composite signals, said apparatus comprising: first and second input circuits having respective first and second input terminals and each including all-pass phase-shifting means for deriving from the first and second composite signals first and second composite output signals which are in phase quadrature relationship with one another, first, second, third and fourth output channels, means for coupling the first and second composite signals from the first and second input terminals without relative phase-shift to said first and second ouptut channels, respectively, first summing means for combining equal proportions of the first composite output signal from the first input circuit and of the second composite output signal from the second input circuit to produce a third composite signal predominantly containing the signal component c, second summing means for combining equal proportions of the second composite signal from the first input circuit and of the first composite output signal from the second input circuit to produce a fourth composite signal predominantly containing the signal component d, and means for coupling said third and fourth composite signals to said third and fourth output channels, respectively.
 10. Apparatus in accordance with claim 9, wherein said input circuits each include a pair of all-pass phase-shifting networks to both of which the corresponding composite input signal is applied, one of the phase-shifting networks of the pair being operative to shift the phase of the applied signal by a predetermined reference angle and the other phase-shifting network of the pair being operative to shift the phase of signals applied thereto by an angle which differs by 90* from the said reference angle.
 11. Apparatus in accordance with claim 10, wherein for each of the composite output signals, the proportion of the signal applied to the first and second summing means is 0.707.
 12. Apparatus for decoding first and second input composite signals respectively designated LT and RT and each including to the extent they are present sub-dominant left back (Lb) and right back (Rb) component signals with the same predetermined phase-shift angle between said Lb component signals and between said Rb component signals and with the Lb component signal in one of said composite signals leading the Lb component signal in the other and with the Rb component signal in said one composite signal lagging the Rb component signal in the other, and respectively including to the extent they are present dominant left front (Lf) and right front (Rf) component signals, into four composite output signals respectively containing said Lf, Rf, Lb and Rb component signals as its predominant signal, said apparatus comprising: first and second input terminals to which said LT and RT signals are respectively applied, first, second, third and fourth output channels respectively identified as Lf, Rf, Lb and Rb output channels, means connected between said first and second input terminals and said first and second output channels for coupling said first and second input composite signals respectively containing said Lf and Rf component signals as predominant signals to said first and second output channels, means connected to said first and second input terminals including all-pass phase-shifting networks for shifting the phase of one of said input composite signals relative to the other by said predetermined phase-shift angle for positioning the Lb and Rb components in one of said relatively phase-shifted composite signals either in phase coincidence or in phase opposition with the Lb and Rb components, respectively, in the other relatively phase-shifted composite signal, and means connected in circuit between said phase-shifting means and said third and fourth output channels for selectively combining predetermined proportions of said relatively phase-shifted first and second composite signals and operative to couple to said third and fourth output channels composite signals which respectively contain said Lb and Rb component signals as predominant components.
 13. Apparatus in accordance with claim 12, wherein said predetermined phase-shift angle is substantially 90*, and wherein said predetermined propoRtion is substantially the decimal fraction 0.707.
 14. Apparatus in accordance with claim 12, wherein said input circuits each include a pair of all-pass phase-shifting networks to both of which the corresponding composite input signal is applied, one of the phase-shifting networks of the pair being operative to shift the phase of the applied signal by a predetermined reference angle and the other phase-shifting network of the pair being operative to shift the phase of signals applied thereto by an angle differing from said reference angle by said predetermined phase-shift angle, and wherein said signal-combining means comprises first summing means for combining equal proportions of the composite signal from the first network of the first pair and the composite signal from the second network of the second pair and coupling to said Lb output channel a composite signal which contains said Lb component signal as its predominant component, and second summing means for combining equal proportions of the composite signal from the second network of the first pair and the composite signal from the first network of the second pair and coupling to said Rb output channel a composite signal which contains said Rb component signal as its predominant component.
 15. Apparatus in accordance with claim 14, wherein said predetermined phase-shift angle is substantially 90*, and wherein for each of the composite signals from said phase-shifting networks, the proportion of the signal applied to the first and second summing means is 0.707.
 16. Apparatus in accordance with claim 14, wherein said predetermined phase-shift angle is substantially 90*, wherein for each of the composite signals from the first network of the first pair and from the second network of the second pair, the proportion of the signal applied to the first summing means is 0.707, and wherein for each of the composite signals from the second network of the first pair and from the first network of the second pair, the proportion of the signal applied to the second summing means is -0.707.
 17. Apparatus in accordance with claim 12, wherein said input circuits each include first and second all-pass phase-shifting networks to both of which the corresponding composite input signal is applied, said first phase-shifting network being operative to shift the phase of the applied signal by a predetermined reference angle and said second phase-shifting network being operative to shift the phase of the applied signal by an angle differing from said reference angle by substantially 90*, wherein the composite signals from the second network of said first and second input circuits, respectively containing said Lf and Rf component signals as their predominant component, are respectively coupled to said first and second output channels, and wherein said signal-combining means comprises first summing means for combining substantially 0.707 of the composite signal from said second network of said first input circuit and substantially 0.707 of the composite signal from said first network of said second input circuit and coupling to said third output channel a composite signal which contains said Lb component signal as its predominant component, and second summing means for combining substantially 0.707 of the composite signal from said first network of said first input circuit and substantially 0.707 of the composite signal from said second network of said second input circuit and coupling to said fourth output channel a composite signal which contains said Rb component signal as its predominant component.
 18. A method of substantially reproducing at least four individual audio information signals respectively designated left front (Lf), right front (Rf), left back (Lb) and right back (Rb) contained in two information cHannels, wherein the signals in said channels respectively include one of said Lf and Rf signals together with predetermined proportions of both said Lb and Rb signals with the same predetermined substantially constant phase-shift angle between said Lb signals and between said Rb signals such that the Lb signal in one of said channels leads the Lb signal in the other and the Rb signal in said one channel lags the Rb signal in the other, comprising the steps of: coupling the signals in said channels to respective output channels without relative phase shift therebetween to provide two composite output signals respectively containing said Lf and Rf signals as predominant components, shifting the phase of the signal in one of said channels relative to the signal in the other channel by said predetermined phase-shift angle for positioning the Lb and Rb components in one channel either in phase coincidence or in phase opposition with corresponding components in the other channel, and selectively combining predetermined equal proportions of said relatively phase-shifted signals to produce third and fourth composite output signals respectively containing said Lb and Rb signals as predominant components.
 19. The method according to claim 18, wherein said predetermined phase-shift angle is substantially 90*, wherein the signals in the two channels are relatively shifted in phase by substantially 90*, and wherein said predetermined proportion is substantially the decimal fraction 0.707.
 20. Apparatus for reproducing at least four individual audio information signals to the extent they are contained in two information channels, wherein the signal in each of said channels comprises a combination of at least three of said audio information signals with preselected phase and amplitude relationships, said apparatus comprising first and second input circuits to which the signals in said two information channels are respectively applied, said input circuits including all-pass phase-shifting means operative to shift the phase of one of said channel signals relative to the other channel signal by a preselected angle which remains substantially constant over the frequency range of said audio information signals thereby to produce relatively phase-shifted first and second channel signals, electrical circuit means for combining said relatively phase-shifted first and second channel signals with preselected amplitude and phase relationships for deriving a first pair of output signals in each of which a different desired one of two of said audio information signals is predominant, and circuit means for deriving from said first and second channel signals a second pair of output signals in each of which a different desired one of the other two of said audio information signals is predominant, whereby a different desired one of said audio information signals is predominant in each of said four output signals.
 21. Apparatus in accordance with claim 20 wherein said phase-shifting means is operative to shift the phase of one of said channel signals relative to the other by substantially 90*.
 22. A decoder for reproducing at least first, second and third of first, second, third and fourth audio information signals to the extent said four audio information signals are contained in first and second composite input signals respectively containing said first and second audio information signals in equal predominant proportions and both containing said third and fourth audio information signals in equal sub-dominant proportions with said third and fourth audio information signals in one of said composite input signals in substantially quadrature relationship with the third and fourth audio information signals, respectively, in the other composite input signals, said decoder comprising: means for coupling said first and second composite signals without relative phase shift between them to first and second output terminals, respectively, as first and second output signals in which said first and second audio information signals are respectively predominant, means for shifting the phase of one of said composite input signals relative to the other by about 90* for positioning the third audio information signal in the phase-shifted one input signal in phase with the third audio information signal in the unshifted other input signal and for positioning the fourth audio information signal in the phase-shifted one input signal in phase opposition with the fourth audio information signal in the unshifted other input signal, and means for combining said phase-shifted one input signal and said unshifted other input signal to produce at a third output terminal a third output signal in which said third audio information signal is predominant.
 23. Apparatus in accordance with claim 22 wherein said predominant and sub-dominant proportions are in the ratio of about 1:.707, wherein said phase-shifting means comprises first and second all-pass phase-shifting networks to which said first and second composite input signals are respectively applied for shifting the phase of one of said composite input signals relative to the other by about 90*, and wherein said combining means comprises first means for adding substantially 0.707 of the output signal from one of said first and second phase-shifting networks to substantially 0.707 of the output signal from the other for producing said third output signal, and second means for subtracting substantially 0.707 of the output signal from one of said first and second phase-shifting networks from substantially 0.707 of the output signal from the other for producing at a fourth output terminal a fourth output signal in which said fourth audio information signal is predominant.
 24. Apparatus in accordance with claim 22, wherein said predominant and sub-dominant proportions are in the ratio of about 1:.707, wherein said phase-shifting means comprises first and second pairs of all-pass phase-shifting networks, the first network of each pair being operative to shift the phase of signals applied thereto relative to the signals applied to the second network of the pair by about 90*, and wherein said first and second composite input signals are applied to both phase-shifting networks of a respective pair, and wherein said combining means comprises a first network for combining substantially 0.707 of the output signal from the second phase-shifting network of the first pair with substantially 0.707 of the output signal from the first phase-shifting network of the second pair for producing said third output signal in which said third audio information signal is predominant, and a second network for combining substantially 0.707 of the output signal from the first phase-shifting network of the first pair with substantially 0.707 of the output signal from the second phase-shifting network of the second pair for producing a fourth output signal at a fourth output terminal in which said fourth audio information signal is predominant.
 25. Apparatus in accordance with claim 24, wherein the first network of each of said pairs is operative to shift the phase of signals applied thereto by a predetermined reference angle and the second network of each of said pairs is operative to shift the phase of signals applied thereto by an angle differing from said reference angle by about 90*. 